Genetic studies of photoperiod response genes and their effect on heading time in Japanese wheat cultivars

Wheat (Triticum aestivum L.) should be harvested before the rainy season in Japan. Therefore, early heading is one of the most important traits in wheat breeding. However, shorter growth periods generally result in lower grain yields, and early-heading wheat cultivars with early apical development and stem elongation are prone to frost injury. Heading time of wheat is a complex characteristic determined by three factors: narrow-sense earliness (also termed earliness per se), vernalization response, and photoperiod response. For fine tuning of heading time in wheat breeding, genetic factors controlling these characteristics should be combined properly. In this study, genotypes of the photoperiod response genes, Ppd-A1, Ppd-B1 and Ppd-D1, in 240 Japanese cultivars were determined using PCR-based methods, and the effect of Ppd-1 genotype on heading time was discussed. In addition, the introduction of photoperiod-insensitive alleles into Japanese cultivars was discussed. The distribution of photoperiod-insensitive alleles differed among Ppd-1 genes, as well as among geographic areas. Most Tohoku-Kyushu cultivars (97.5%) carried Ppd-D1a, and 10 cultivars, including three commercial extra-early cultivars, carried both Ppd-B1a and Ppd-D1a, while no cultivars carried Ppd-A1a. Among Hokkaido winter wheat cultivars, 41.4% and 24.1% carried Ppd-A1a and Ppd-D1a, respectively, and none of the cultivars carried Ppd-B1a. In contrast, in Hokkaido spring wheat cultivars, only one experimental line carried Ppd-D1a, and the other cultivars did not carry photoperiod-insensitive alleles. The effect of Ppd-1 alleles on heading time also differed among areas. In the Tohoku-Kyushu region, wheat cultivars carrying the photoperiod-insensitive allele Ppd-D1a headed earlier by 10.3 days than did photoperiod-sensitive cultivars, and Ppd-B1a/Ppd-D1a genotype headed earlier by 6.7 days than did Ppd-B1b/Ppd-D1a genotype. In the Hokkaido region, photoperiod-insensitive cultivars with Ppd-A1a or Ppd-D1a headed earlier by 2.5 days than did photoperiod-sensitive cultivars with no photoperiod-insensitive alleles, although the effect of these alleles was less than that in the Kanto region. The geographical difference of distribution and the effect of photoperiod-insensitive alleles in this study may result from the difference of day-length of the wheat apical spike formation stage. Pedigree analysis of extra-early wheat cultivars showed that Ppd-B1a in three extra-early commercial cultivars was inherited from 'Shiroboro 21' by early-heading Chugoku lines, bred at the Chugoku Agriculture Experimental Station. In Japan, except in Hokkaido, the rainy season starts before the wheat harvest; thus, early cultivars with Ppd-D1a have been selected to avoid damages such as preharvest sprouting and Fusarium head blight. Furthermore, it is suggested that the introduction of the Ppd-B1a accelerated early-maturity wheat breeding in Japan. Three cultivars with Ppd-B1a/Ppd-D1a genotype are successfully established only in the Chugoku or Kyushu region. It was considered that such cultivars often suffered from late frost because of early stem elongation in eastern Japan, like the Kanto region, where the daily minimum temperature was below freezing in the winter. Besides, heading time of extra-early cultivars is variable depending on winter temperature, and causes instability of grain yield. However, heading time of extra-early cultivars can be stabilized by the introduction of adequate vernalization requirements. Pedigree analysis of Hokkaido winter wheat cultivars showed that 'Purple Straw' and 'Tohoku 118' were one of the donor(s) of Ppd-A1a and Ppd-D1a in Hokkaido wheat cultivars, respectively. Wheat cultivars recently developed in Hokkaido carry photoperiod-insensitive alleles at a high frequency. For efficient utilization of Ppd-1 alleles in the Hokkaido wheat-breeding program, the effect of Ppd-1 on growth pattern and grain yield should be investigated. The results in this study clearly indicated that heading time varied largely even among the Japanese spring type cultivars except those in the Hokkaido region, in spite of having the same set of photoperiod response and vernalization response genes. It was suggested that their heading time is affected by other photoperiod response genetic factor(s) and other photoperiod insensitive allele(s) of Ppd-1. In addition, variation in heading time could be partly explained by narrow-sense earliness, since it is also important for the control of heading time in Japanese wheat cultivars. Further advancement of molecular genetics of narrow-sense earliness should be expected, and would make it possible to discuss the significance of this characteristic in Japanese wheat breeding.